1. Field of the Invention
The method of the present invention relates to the field of seismic data interpretation for the purpose of locating a surface in a three dimensional volume of seismic data. The present invention applies mathematical gridding to the location of surfaces in a volume.
2. Description of the Prior Art
In the field of oil and gas exploration, locating surfaces within a seismic volume is essential for deciding where exploratory wells are to be drilled and for understanding the geologic history of the rock represented by the seismic volume. Interpreters in the oil and gas industry typically spend months locating and marking surfaces in such a volume. The process of marking the locating and marking surfaces in a seismic volume is called xe2x80x9cseismic interpretation,xe2x80x9d and an xe2x80x9cinterpreterxe2x80x9d is one who does this job. xe2x80x9cInterpretingxe2x80x9d is also known as xe2x80x9cpicking,xe2x80x9d and the xe2x80x9cpicksxe2x80x9d are typically referred to as xe2x80x9chorizons.xe2x80x9d
Subsurface rocks are generally porous, like beach sand but with less ability to absorb fluids, and contain either water, oil, or gas. Oil and gas are lighter than water and float upward in the subsurface as they do at the surface. The path of movement and the cessation of movement are in large part dependent on the geometry of the subsurface layers in which the fluids move, which makes the location of surfaces in seismic data of interest to interpreters.
In addition to defining the geometric relationships of the surfaces, seismic data may reveal the location of ancient seabeds, streams, reefs, and other features where organic matter may accumulate to become oil or gas or where oil or gas may be trapped. Such information has great utility in geophysical exploration for hydrocarbons.
Oil and gas are for the most part found thousands of feet below the surface of the Earth. Determining where oil and gas form and accumulate requires understanding features seen at these depths. Much of what exists currently at depth was once at the Earth""s surface, either at the dry land surface or at the seabed or below some other body of water. Understanding these buried surfaces begins with obtaining their shapes and relating the shapes to one another.
Much of the search for hydrocarbons is conducted using seismic data. Seismic data are produced by transmitting an acoustic signal (generated by dynamite, for example) into the Earth and recording echoes of this signal. The layers of rock within the Earth differ in their acoustic properties, and these changes in property produce echoes of the seismic signal. The reflecting surfaces generally mimic the surface of the Earth at some time in the past, so it is the mapping of these surfaces that occupies a significant portion of the exploration process.
FIG. 1 depicts a typical seismic echo as detected by a receiver at the Earth""s surface. It is a sinusoidal curve as a function of time. The strength of the echo rises and falls over a period of several seconds, and this rise and fall is recorded in digital form or converted to digital form for processing and analysis. A single echo train is usually called a xe2x80x9cseismic tracexe2x80x9d.
The echo train consists of amplitude characteristics of interest such as xe2x80x9cpeaksxe2x80x9d and xe2x80x9ctroughsxe2x80x9d. Peaks, by convention, deflect to the right and are black-filled in this display. Troughs deflect to the left and are not filled in this display; only the xe2x80x9ctracexe2x80x9d, or single line, of the deflection is present for troughs. The trace crosses the zero-amplitude line where the black fill of the peaks form a vertical boundary. The points of crossing are called xe2x80x9czero crossingsxe2x80x9d. Zero crossings are another example of amplitude characteristics of interest. Some zero crossings transition from peaks to troughs and others transition from troughs to peaks. In a general way each peak and trough represents a reflecting boundary, so that each trace provides a view of the subsurface at its location.
When such traces are collected along a line on the surface, an image such as shown in FIG. 2 can be created. A collection such as is represented by FIG. 2 is called a xe2x80x9cseismic sectionxe2x80x9d or a xe2x80x9csectionxe2x80x9d because it is a cross-section through the Earth as represented by seismic traces. One can clearly see that there is a pattern to the reflection surfaces along this section. Peaks and troughs line up laterally in an organized way.
The interpreter""s job is to decide which peaks or which troughs are to be tracked laterally and then to make a record of the tracking by picking horizons. Sometimes peaks change to troughs laterally, and it is the interpreter""s job to decide whether such a change means the event has terminated, or has shifted up or down, or continues as the reversed feature. Such transitions can be gradual, and the interpreter must decide where the event is on the other side of the transition. Sometimes zero crossings are to be tracked. In the usual case the event of interest does not transition to its opposite, but the fading and re-emergence of an event can be common.
Prior art patents in the field of seismic data interpretation disclose methods for searching or interpolating from one seismic trace to adjacent seismic traces. Such methods are disclosed in U.S. Pat. Nos. 5,153,858 and 5,432,751 to Hildebrand; U.S. Pat. Nos. 5,251,184 and 5,615,171 to Hildebrand et al.; and U.S. Pat. No. 5,056,066 to Howard. These prior art patents do not disclose a method that employs a mathematical surface for locating a surface in a three dimensional volume of data.
The present invention is directed toward a method of locating a surface in a three dimensional volume of seismic data. In the present invention, an acoustic signal is transmitted into the Earth to produce a multiplicity of reflected acoustic signals that are received by at least three non-colinear receivers. These receivers define a grid having an x axis dimension and y axis dimension. Amplitude values of each reflected acoustics signal are recorded as a function of time to construct a seismic volume comprising a seismic trace for each recorded acoustic signal. The seismic volume comprises a z axis dimension that measures the time of each reflected acoustic signal.
The present invention is directed toward a method of calculating a set of great coordinates of the type mi=(xi, yi, ti), based upon the location of an amplitude characteristic of interest.
The present invention permits the use of algorithms known to those of ordinary skill in the art to define a mathematical surface from the data in the seismic volume. The defined mathematical surface is then used to autopick additional data points needed to locate a surface of interest in the seismic volume.